1. Field of the Invention
High-temperature alloys of high resistance to oxidation and corrosion, based on intermetallic compounds, which are suitable for directional solidification and supplement the conventional nickel-based superalloys.
The invention relates to the further development and improvement of the alloys based on the intermetallic compound Ni.sub.3 Al with further additives which increase the hot strength and oxidation resistance.
In a narrower sense, it relates to an oxidation- and corrosion-resistant high-temperature alloy of high toughness at room temperature for directional solidification, based on an intermetallic compound of the nickel aluminide type
2. Discussion of Background
The intermetallic compound Ni.sub.3 Al has some interesting properties which make it appear to be attractive as a material of construction in the medium temperature range. These include, inter alia, its low density as compared with superalloys. Its usability in engineering in the present form is, however, prejudiced by its brittleness and its inadequate corrosion resistance. Although the former can be improved by additions of boron, higher strength values also being obtained (cf. C.T. Liu et al., "Nickel aluminides for structural use", Journal of Metals, May 1986, pages 19-21), this procedure has nevertheless not led to any results useful in practice in strip production, even when high cooling rates are applied.
The corrosion resistance and oxidation resistance of such alloys based on Ni.sub.3 Al can be improved by additions of silicon or chromium (cf. M.W. Grunling and R. Bauer, "The role of Silicon in corrosion resistant high temperature coatings", Thin Films, volume 95, 1982, pages 3-20). In general, alloying with silicon is a more suitable approach than that with chromium, since the intermetallic compound Ni.sub.3 Si, arising simultaneously, is fully miscible in Ni.sub.3 Al. These are thus isomorphous states, no further, undesired phases being formed (cf. Shouichi Ochiai et al., "Alloying behaviour of Ni.sub.3 Al, Ni.sub.3 Ga, Ni.sub.3 Si and Ni.sub.3 Ge", Acta Met. volume 32, no. 2, page 289, 1984).
The hot strength of Ni.sub.3 Al and of the above modified alloys is, however, still inadequate, as is clear from publications on intermetallic compounds (cf. N.S. Stoloff, "Ordered alloys--physical metallurgy and structural applications", International Metals Review, volume 29, no. 3, 1984, pages 123-135).
It is known that, inter alia, silicon increases the corrosion resistance and oxidation resistance of surface layers forming protective oxides in coatings of high-temperature alloys. Extensive investigations have been carried out on this point (cf. F. Fitzer and J. Schlichting, "Coatings containing chromium, aluminum and silicon for high temperature alloys", High temperature corrosion, National Association of Corrosion Engineers, Houston, Tex., and San Diego, Calif., Mar. 2-6, 1981, pages 604-614).
The properties of these known modified Ni.sub.3 Al materials in general still do not meet the engineering requirements for producing useful workpieces from them. This applies in particular with respect to hot strength and high-temperature corrosion resistance (resistance to sulfidation) as well as ductility and toughness at room temperature. There is therefore a demand for further development and improvement of such materials.